Until recently, scientists were puzzled why an earthquake of only 7.5 magnitude caused a devastating tsunami that killed over 2,000 people on the island of Sulawesi in Indonesia in September of 2018. The earthquake didn’t even trigger the tsunami early warning system as normally a tsunami of the size (wave height up to 11m or 36 ft) would require an 8.4 or higher magnitude earthquake. A new study published in the journal Pure and Applied Geophysics has found that the cause of the tsunami was actually a marine landslide in Palu Bay on the west coast of Sulawesi. A combination of computer modeling and observation of the actual sea level data from the event was used to reach their conclusion. They observed a wave period that was very short, consistent with the 3 to 4 minutes expected based on the modeling of a marine landslide compared to the 15 to 60 minute periods for an earthquake-induced tsunami.
The Chilliwack Times reports that the City of Chilliwack will pay 80% of the assessed home value for up to 42 homes in an Eastern Hillside subdivision that are located on a slow moving landslide. The issue was first noticed in 2001 and several homes have had significant damage, but most are currently undamaged. The City denies any responsibility, but it’s legal counsel recommended a settlement. (Photo by Paul J. Henderson, Chilliwack Times)
Normally the geotechnical engineer for the subdivision would be held accountable. But apparently the slip surface is located 30-m below grade, much deeper than borings for a typical investigation for a subdivision. I wonder if there were any geologists consulted? Click through for a Google map view of the area and you can do your own armchair photogeology quarterbacking! Your heart does go out to the people losing their homes, they had no idea. But it could be worse, they could be in La Jolla and be getting squat.
The Devil’s Slide Tunnel project is on schedule and on budget according to a news story at ABC7News.com from earlier in June. The video (shown after the break) has a few nice shots showing rock bolting, soil nailing at the portals, and the geologic mapping and laser scanning that happens at the tunnel face.
I also came across a very neat article about how the Ocean Shore Railroad Company was the first to try to cut into the slope along what is now PCH 1 at the Devil’s Slide back in the early 1900s. They were trying to connect the then rural farming community of Half-Moon Bay with San Francisco. The railroad fought the reoccurring landslide and serious rockfalls. Ocean Shore Railroad went bankrupt in 1922 and pulled up its rails, making room for the current highway. (Photo at left from halfmoonbaymemories.com)
Residents of approximately 75 of the 111 homes evacuated after the slide have been allowed back into their homes. The City of San Diego is not wasting any time in determining the cause of the slide. The first of three 100-ft deep exploration shafts was excavated on Friday, and a Forensic Geologist from a firm hired by the City was sent down the hole to observe geology conditions and to locate the actual slip surface. Also, claims of leaking City water and/or sewer pipes prior to the failure have begun to surface. More details in the full post. (Photo by NELVIN CEPEDA / San Diego Union-Tribune)